Contact lenses of soft or hard type (hereinbelow collectively designated as “contact lenses”) are typically furnished in the center part of the lens with an optical zone that affords certain optical characteristics, and a peripheral zone provided about the circumference of the optical zone, for stabilizing lens shape. The optical zone and the peripheral zone are connected by a joint zone.
In the case of toric contact lenses for correcting astigmatism, it will be necessary to prescribe a lens provided not just with the required spherical diopter power, but also with appropriate cylindrical power and cylindrical axial angle depending on the slope of the astigmatic axis of the wearer. Accordingly, in the case of contact lenses such as those used to correct astigmatism, the optical characteristic of the optical zone will need to be established in a manner inclusive not only of spherical diopter power, but also of elements requiring positioning in the circumferential direction, such as cylindrical power and cylindrical axial angle.
Where the required optical characteristics of contact lenses in the circumferential direction must vary depending on the wearer, the profiles of the optical zone must vary as well. Thus, there will be disparities in the circumferential distribution of a junction thickness in the joint zone. For example, in the case of a toric lens with a cylindrical axial angle of 90°, when worn in the eye the two horizontal edges of the optical zone will be thicker, whereas in the case of a toric lens with a cylindrical axial angle of 180° the two vertical edges of the optical zone will be thicker. Consequently, it will be more difficult to achieve good wear comfort and circumferential stability with a 180° cylindrical axial angle lens as compared with a 90° cylindrical axial angle lens. Further, in the case of a lens having a cylindrical axial angle of 45° for example, irregular torque will tend to arise due to the asymmetrical sideways profile, and circumferential stability will tend to be poor. Thus, for a lens having given spherical diopter power and cylindrical power, appreciable disparities in the circumferential distribution of the junction thickness in the joint zone may arise due, for example, to different cylindrical axial angles. Depending on the individual, such differences may result in the problem of inability to achieve the desired level of wear comfort and circumferential stability. Additionally, as progressively greater levels of cylindrical power are established, the difference in curvature ratio between the optical zone and the peripheral zone will become greater as well, so the joint zone will tend to inflect, which tends to result in diminished wear comfort.
To address such problems, Patent Citation 1 (Japanese PCT Republication No. JP-A-2001-519046) for example discloses a method whereby an appropriate optical zone diameter is selected according to the cylindrical power in order to minimize disparities in thickness dimension among lenses of differing cylindrical power. However, with contact lenses like that taught in Patent Citation 1, disparities in the circumferential location of thickness will arise among lenses with different cylindrical axial angle settings, for example. As noted, differences in circumferential location of thickness have appreciable effect on differences in wear comfort and circumferential stability, and thus with the method taught in Patent Citation 1 there is a risk that, depending on the cylindrical axial angle, inability to achieve the desired level of wear comfort and circumferential stability will be a problem.
Patent Citation 2 (Japanese Patent No. 2695056) discloses a contact lens with reduced inflection of the joint zone by forming within the joint zone a transition portion that affords smooth connection between the optical zone and the peripheral zone. However, the contact lens taught in Patent Citation 2 merely has reduced inflection in the joint zone, and as in Patent Citation 1, such a lens will still entail differences in circumferential location of thickness of the sort discussed previously. Thus, as in Patent Citation 1, with the contact lens of Patent Citation 2, an inability to achieve the desired level of wear comfort and circumferential stability may be a problem.
Further, Patent Citation 3 (U.S. Pat. No. 5,125,728) discloses a contact lens in which an optical zone anterior face and an optical zone posterior face having specific profiles are combined in such a way as to provide improved wear comfort and circumferential stability. However, as in Patent Citations 1 and 2, the contact lens of Patent Citation 3 will still entail differences in circumferential location of thickness arising for disparities in the cylindrical axial angle for example. Thus, depending on the setting location of the cylindrical axial angle, an inability to achieve the desired level of wear comfort and circumferential stability may be a problem.    Patent Citation 1: Japanese PCT Republication No. JP-A-2001-519046    Patent Citation 2: Japanese Pat. No. 2695056    Patent Citation 3: U.S. Pat. No. 5,125,728